Electric heaters



Feb. 11, 1964 J, A, MENzlEs ETAL 3,121,154

ELECTRIC HEATERS l ,6 F/G.3. &\W N'/ A INVENToRs l -James A. Menzles BYGeorge D. Robertson ATTORNEY Feb. 11, 1964 Filed oct. so, 1959 J. A.MENZIES ETAL ELECTRIC HEATERS n [5| aq/. ww(

5 Sheets-Sheet 2 INVENToRs James AI Menzles iBY George D. Rober'rsor)ATTORNEY Feb. 11, 1964 J. A. MENzlEs ETAL 3,121,154

ELECTRIC HEATERS Filed Oct. 30, 1959 3 Sheets-Sheet 3 F/GS,

INVENT ORS ATTORNEY United States Patent O 3,121,154 ELECTRIC HEATERSJames A. Menzies, London, England, and George D. Robertson, Glasgow,Scotland, assignors to Babcock & Wilcox, Limited, London, England, acorporation of Great Britain Filed Oct. 30, 1959, Ser. No. 849,775 3Claims. (Cl. 219-19) This invention relates to electric heaters. ln ahighly rated electric heater of the kind including an electricresistance and a covering means, for example, an enclosing sheath,difficulty is experienced in insulating the resistance from the sheathin a manner adapted to withstand the relatively high temperature and topermit high heat transfer rates to the covering means |when the latteris required to operate at temperature of the order of 900 F. The presentinvention enables highly rated electric heaters to be provided whichhave satisfactory insulation enabling particularly high rates of heattransfer from the resistance to the covering means to be achieved.

The present invention includes an electric heater having an electricresistance `and a covering means for the electric resistance wherein theresistance is insulated from the covering means by a thin, dense andcoherent layer of oxidized aluminum.

ln a form of rod heater in accordance with the invention, the resistanceis of helical form, its radially outer surface being in contact with aninterior anodized aluminu-m film on a surrounding tube which may be ofaluminum alloy. Suitably the helix is in the form of a ribbon with thefaces of the ribbon facing radially inwards and radial-ly outwards. Theribbon may be of uniform area and shape, referred to cross-sectionstransverse to the ribbon, or, if it is desired that the heat outputtherefrom shall vary from point to point yalong its length, thecrosssectional area may appropriately 'vary from point to point alongits length, by variation, for example, of the radial dimension only ofthe ribbon along its length, the radial Kdimension being reduced inregions in which it is desired that the heat output per unit length ofthe heater shall be relatively greater.

The various features of novelty which characterize our invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of -theinvention, its operating advantages and specific objects attained by itsuse, reference should be had to the accompanying -drawings anddescriptive matter in which we have illustrated and described preferredembodiments of the invention.

Of the drawings:

FIGURE 1 is :an elevation partly in section on its axis of one form ofthe invention;

FIGURES 2 and 3 are sectional elevations of parts of modified forms ofthe heater shown lin FIGURE l;

FIGURE 4 is an elevation of a tubular heater, partly in a section on itsaxis; and

FIGURE 5 is a section through a heater unit comprising `an assembly ofheater plates.

Referring to FIGURE l of the drawings, -a heater for simulating a fuelelement for a gas-cooled, natural uranium nuclear reactor core comprisesa covering means such `as aluminum alloy tube 10, of the same length andoutside diameter as the length and outside diameter of the can of thefuel element and housing a helical electric resistance heating element12.

The helical heating element is formed by machining a helical gap 13along the length of a tube of suitable metal, for example, .theiron-chromium-aluminum-cobalt alloy known as Kanthal A, or a stainlesssteel of a kind used for resistance heaters, to `form a helical ribbonwith 3,121,154 Patented Feb. l1, 1964 the faces of the ribbon facingradially inwards and radially outwards. At its ends the ribbon is brazedor silver soldered to respective brass cylindrical blocks 14 which closethe respective ends of the surrounding tube and also act as terminalsthrough which the ribbon may be connected into an appropriate electriccircuit.

The interior surface of the covering tube 10 is covered by a continuous,dense and coherent film 15, having a uniform thickness of the order of`0.002 inch, formed electrolytically by the treatment development forproducing a particularly Wear-resistance surface on the alloy containinga sufficiently large proportion of aluminum and known as ha-rdanodizing. The edges 16 bounding the said interior surface arerounded-off and the anodized film extends a certain distance onto therounded edges 16, whereby the tube surface uncovered by the film isspaced from the blocks 14 by a considerably greater distance than thethickness of the film.

'Ihe outside diameter of the tube from which the ribbon 12. is formed isslightly greater than the internal diameter of the covering tube 10 withits anodized film 15, e.g. the said outside diameter may be at leastthat of the internal diameter of the covering tube before the anodizedfilm is formed; for this reason the helical ribbon, in place in thecovering tube, is under strain and the radially outer face 17 of theribbon is forced into intimate contact with the .anodized film 15. Wheninserting the ribbon into the covering tube, one cylindrical block 14 istemporarily turned with respect to the other so that the ribbon assumesa reduced diameter, whereby the longitudinal movements of the coils ofthe ribbon inside the covering tube easily move to take up positions inwhich the coils are evenly spaced. The diameter of the cylindricalblocks is substantially the lsame as the internal diameter of thecovering tube before the anodized film is formed.

In the operation of the heater, heat generated by resistive heating ofthe coils of the ribbon 12 passes outwardly from the coils through theanodized film 15 and the wall of the covering tube 10 to whatever bodyis provided for accepting the heat. The reference numeral 18 denotessome of a number of fins, which could be integral with the coveringtube, which might be provided vto assist in the transfer of heat to acooling gas iiowing tween the surfaces intimately contacting it on itsinner and outer faces respectively, whereby the heating ribbon runs atminimum temperature, but is thick enough to act as electricalinsulation; the anodized film retains its electrically insulatingproperties in spite of high temperatures.

In a rod heater of the nature described, having -a covering tube of15/16 inches external diameter, it has been found possiblesatisfactorily to effect heaty transfer at the rate of l5 kilowatts perfoot length of heater with the covering tube at a temperature of about900 F.

An oxidized aluminum film adapted for the ready passage therethrough ofheat in large quantities while acting as electrical insulation may bedeposited on metal surfaces by a suitable known spraying process. Thus,instead of an anodized film 15 on the interior surface of the coveringtube 1t) in the rod heater described with reference to FIGURE l, anoxidized aluminum film may be used which has been deposited by sprayingon the radially outer face 17 of the helical ribbon 12, and also on thecylindrical surface of the cylindrical blocks 14, insulating the blocksfrom the covering tube. In this modification it is appropriate toround-off, in the manner illustrated in FIGURE 2, the edges 19 of thehelical ribbon 12 bounding the radially outer face 17 of the ribbon, and

:spectively or on the heating element.V

4the electric resistance heating element is in the form of a tube 22 ofwhich the radially outer surface 27 is in *Contact with a hard anodizedfilm which has been formed on the interior surface of the covering tubelil. The tubular heating element 22 and the covering tube 10 are of thesame length; the terminals are constituted by cylindrical blocks 14closing the respective ends of the tubular heating element, which blocksmay be tackbrazed or tack-silver soldered to the tubular heating elementafter having been driven or screwed into position. 4The edges 16bounding the interior surface of the tube 10 are rounded and theanodized film 1S extends a certain distance onto the rounded edges 16.If the tubular heating element 22 makes a push fit in the surroundingtube 1t) before the anodized film 15 has been formed on the interiorsurface of the latter, the tubular heating element may, after the hardanodized film of the order of 0.002 inch thick has been formed be driveninto position within the surrounding tube 10 without damagaing thedesired properties of the film and intimate contact relsults between theradially outer surface V27 of the tubular heating element 22 and theinner surface of the film ,'15, Alternatively, if desired, the tubularresistance element 22 may be expanded into intimate contact with thefilm 15, the expansion being made by hydraulic or meuchanical means ortaking place by warming up after it has been inserted into the coveringtube in chilled condition, or the outer tube may be shrunk about theresistance element by any of the means well known in the art.

FIGURE 4 shows a tubular heating element 31 of Kanthal A or stainlesssteel mounted in a tubular heater adapted for heating fluid flowingthorugh a central passage 32 of the heater as well as fluid flowing overthe outside of the heater. The tubular heating element is held at itsends in respective similar terminal block assemblies 33 each consistingof an annular inner brass block 34 and an annular outer brass block 35.Each end length of the heating element lits over a cylindrical outersurface 36 of the appropriate inner block 34 and within a cylindricalinner surface 37 of the appropriate outer block 35, and the extremitythereof is brazed or silversoldered as at 39 to the inner end as at 3SVto the outer brass block.

' Intermediate its end lengths the heating element 31 is supportedrby aclosely-fitting inner covering tube 4@ and a closely-fitting outercovering tube 41 both of aluminum or aluminum alloy, which are also heldby the terminal block assemblies, the former tube 4h tting at each endbetween the heating element and an annular collar 42 on the appropriateinner brass block 34, and the latter tube 41 fitting at each end betweenthe heating element and an annular collar 43 on the appropriate outerbrass block 33.

The inner and outer covering tubes 40 and d1 are electrically insulatedfrom the heating element, and also from the terminal block assemblies33, by electrolytically or. spray deposited aluminum oxide layers orim44land 45 on the inner and outer covering tubes 40 and 41 re- Thus thefilm 44 extends over the'whole length of the outer surface, facing theinner surface of the heating element, over the ends 46, suitablyrounded, and over endlengths, longer 1 than the collars 42, of Vtheinner surface of theV inner covering tube 40. The film 45 also extendsover the whole length of the inner surface, facing the heating element,over the ends 037, suitably rounded, and over end lengths, longer thanthe collars 53, of the outer surface of the outer covering tube 41.

Each inner block 34 provides an aperture 43 leading to the passage 32within the tubular heating element. The terminal block assemblies 33serve for leading electric current to and from the heating element, heatfrom which passes through the aluminum oxide films 44 and 45 and throughthe inner and outer covering tubes 40 and 41 to the duid to be heated.

FIGURE 5 is a cross-section of a modified heater umt comprising aplurality of flat heating element plates 81 of aluminum, aluminum alloyor zirconium assembled into a group, parallel to one another, withuniform gaps between their adjacent faces, tot form a heater unitadapted for heating gaseous lluid. The plates have opposite sidesentered into slots 82 equidistantly spaced on opposed faces onrespective support plates 83 of -aluminum or aluminum alloy formingpar-t of or secured to some suitable supporting structure for theheating unit as a whole. O11 said opposite faces of the support platesincluding the surfaces of the slots, a dense, coherent film of aluminaof at least 0.002 inches thick has been formed by a hard anodizingprocess. Alternatively or in addition, when l the resistance elementsare of aluminum or aluminum resistance, a thin dense vand coherent filmof oxidized block assembly.

alloy the end portions 36 of the resistance elements which enter theslots may be formed with hard anodized lms. Electric current may be ledto and from the plate ltype heaters through one or'more terminal stripssuch as 84 at one end of the heater unit each -welded at that end to theedges of all the heating element plates, and a similar terminal strip orstrips at the other end of the heater unit.

While `in accordance with the provisions of the statutes we haveillustrated and described herein the best forms and modes of operationof the invention now known to us, those skilled in the art willunderstand that changes may be made in the forms of the apparatusdisclosed without departing from the spirit of the invention coveredV byour claims, and that certain features of our invention may sometime beused to advantage without -a corresponding use of other features.

We claim:

l. An electric heater having a plurality of electric resistance plates,said resistance plates positioned in paralel relationship to oneanother, a pair of supporting plates oppositely arranged transversely ofsaid resistance plates and on opposite sides thereof and having aplurality of parallel slots on the opposed faces thereof, saidresistlance plates yadapted to fit in said slots in said supportingVtubular resistancea second covering tube interior of said tubularresistance, a thin dense and coherent lm of oxidized aluminum in directcontact with ,and between said resistanceA and said first Vand secondcovering tube,

and at least one terminal block assembly adapted to sup-V port at leastone end of said tubular resistancel and said coveringtubes, said film ofoxidized aluminum further disposed between said covering tubes and saidterminal 3. An electric heater comprising an electrical resistance of`tubular shape, a covering tube enclosingV said aluminum in directcontact withand between said resistance and said covering-tubefandatleast one terminal block assembly adapted to support at least one endrofsaid tubular resistance and said covering tube, said lm of oxidizedaluminum further ldisposed between said covering tube and said terminalblock assembly.

References Cited in the le of this patent UNITED STATES PATENTSAppleyard Oct. 4, 1892 Lohmann Nov. 19, 1929 Fanzer Aug. 9, 1932 FeketeAug. 3, 1937 Doczekal Mar. 8, 1938 Walker NOV. 8, 1938 Duston Nov. 2,1943 6 Osterheld Sept. l2, 1944 Osterheld Oct. 10, 1944 Osterheld Dec.28, 1948 Lennox Mar. 7, 1950 Horsfall et al Mar. 6, 1951 Huck Feb. 21,1956 Glaser May 15, 1956 Hatch Feb. 4, 1958 Norton Feb. 24, 1959 SawyerDec. 8, 1959 Fenu Nov. 8, 1960 Kawalle Aug. 8, 1961

2. AN ELECTRIC HEATER COMPRISING AN ELECTRICAL RESISTANCE OF TUBULARSHAPE, A COVERING TUBE SURROUNDING SAID TUBULAR RESISTANCE, A SECONDCOVERING TUBE INTERIOR OF SAID TUBULAR RESISTANCE, A THIN DENSE ANDCOHERENT FILM OF OXIDIZED ALUMINUM IN DIRECT CONTACT WITH AND BETWEENSAID RESISTANCE AND SAID FIRST AND SECOND COVERING TUBE, AND AT LEASTONE TERMINAL BLOCK ASSEMBLY ADAPTED TO SUPPORT AT LEAST ONE END OF SAIDTUBULAR RESISTANCE AND SAID COVERING TUBES, SAID FILM OF OXIDIZEDALUMINUM FURTHER DISPOSED BETWEEN SAID COVERING TUBES AND SAID TERMINALBLOCK ASSEMBLY.